iio: frequency: adf4350: Fix ADF4350_REG3_12BIT_CLKDIV_MODE

iio: frequency: adf4350: Fix ADF4350_REG3_12BIT_CLKDIV_MODE

The clk div bits (2 bits wide) do not start in bit 16 but in bit 15. Fix it
accordingly.

Fixes: e31166f0fd48 ("iio: frequency: New driver for Analog Devices ADF4350/ADF4351 Wideband Synthesizers")
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250829-adf4350-fix-v2-2-0bf543ba797d@analog.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>